1. Field
The present disclosure relates generally to global navigation satellite systems (GNSS), and more specifically to GNSS surveying receivers using multiple real time kinematic (RTK) engines.
2. Description of Related Art
Navigation receivers that use global navigation satellite systems, such as GPS or GLONASS (hereinafter collectively referred to as “GNSS”), enable the highly accurate determination of the position of the receiver. The satellite signals comprise carrier harmonic signals that are modulated by pseudo-random binary codes and which, on the receive side, are used to measure the delay relative to a local reference clock. These delay measurements are used to determine the so-called pseudo-ranges between the receiver and the satellites. The pseudo-ranges are different from the true geometric ranges because the receiver's local clock is different from the satellite onboard clocks. If the number of satellites in sight is greater than or equal to four, then the measured pseudo-ranges can be processed to determine the user's single point location as represented by a vector X=(x,y,z)T, as well as to compensate for the receiver clock offset.
GNSS finds particular application in the field of surveying, which requires highly accurate measurements. The need to improve positioning accuracies has eventually led to the development of differential navigation/positioning. In this mode, the user position is determined relative to the antenna connected to a base receiver or a network of base receivers, assuming that the positional coordinates of the base receiver(s) are known with high accuracy. The base receiver or receiver network transmits its measurements (or corrections to the full measurements) to a mobile navigation receiver (or rover). The rover receiver uses these corrections to refine its own measurements in the course of data processing. The rationale for this approach is that since the pseudo-range measurement errors on the base and rover sides are strongly correlated, using differential measurements will substantially improve positioning accuracy.
Usually, the base is static and located at a known position. However, in relative navigation mode, both the base and rover are moving. In this mode, the user is interested in determining the vector between the base and the rover. In other words, the user is interested in determining the continuously changing rover position relative to the continuously changing position of the base. For example, when one aircraft or space vehicle is approaching another for in-flight refueling or docking, a highly accurate determination of relative position is important, while the absolute position of each vehicle is generally not critical.
The position of the rover changes continuously in time, and thus should be referenced to a time scale. The determination of the position of a mobile rover with respect to a base receiver in real-time may be performed using a “Real-Time Kinematic” or RTK algorithm, which is stored in memory on the rover. As the name “real time kinematic” implies, the rover receiver is capable of calculating/outputting its precise position as soon as raw data measurements and differential corrections are available at the rover (i.e., practically instantly). The RTK mode uses a data communication link (typically either a radio communication link or a GSM binary data communication link), through which all the necessary information is transmitted from the base to the rover.
Further improvement of the accuracy in differential navigation/positioning applications can be achieved by using both the carrier phase and pseudorange measurements from the satellites to which the receivers are locked. Hone measures the carrier phase of the signal received from a satellite in the base receiver and compares it with the carrier phase of the same satellite measured in the rover receiver, one can obtain measurement accuracy to within a small fraction of the carrier's wavelength
The accuracy of the differential positioning may also depend on the RTK algorithm. For example, the RTK algorithm may make assumptions about, among other factors, how many GNSS satellites to use or how to handle outlier measurements. If these assumptions are correct, the RTK algorithm may quickly produce an accurate position. If these assumptions are incorrect, the RTK algorithm may produce a less accurate position or may take longer to produce an accurate position.